For casual target shooting with my brass framed 1858 I have used 25-30g of fff goex and it has not hurt the gun at all. The load is in the manual that came with the pistol and it doesn't come close to filling the chambers but fills it enough that I need no filler.

I keep my fff T7 loads to 25g and love the way that powder works in the pistol. I ran the Pyrodex P up to 30g did not like the power or the dirt and quit using it.